Generally, a hydraulic machine such as a hydraulic excavator comprises a plurality of hydraulic actuators each of which is controlled by a drive control system including a control lever device. As disclosed in JP, A, 1-97729, for example, one of such drive control systems includes an electric lever device as the control lever device.
The disclosed drive control system is mounted on, e.g., hydraulic excavators for digging earth and sand or the like, and comprises an electric lever device which comprises a control lever operable in respective different operation areas disposed on both sides of a neutral position, a neutral position detecting device for detecting the neutral position of the control lever, and an output device, e.g., a potentiometer, for generating an electric operation signal depending on an input amount of the control lever; control means, i.e., a controller, which comprises calculating means for receiving the operation signal from the electric lever device and for calculating a control signal for a directional control valve corresponding to the received operation signal, and output means for outputting an electric drive signal corresponding to the control signal calculated by the calculating means; a pilot-operated directional control valve, which comprises a hydraulic converter for receiving the drive signal from the output means and converting it into a hydraulic signal, and is connected to a hydraulic circuit for driving a hydraulic actuator and is driven with a hydraulic pilot pressure from a hydraulic source; and a solenoid switching valve for receiving a detection signal from the neutral position detecting device equipped in the electric lever device to selectively cut off the hydraulic pilot pressure between the hydraulic source and the electro-hydraulic converter when the control lever is in the neutral position.
The directional control valve, the electric lever device, and the output means of the controller are each provided plural in number corresponding to a plurality of hydraulic actuators, e.g., a boom cylinder, an arm cylinder, a bucket cylinder, a swing motor and a travel motor.
With the arrangement described above, the disclosed prior art is primarily intended to ensure safety of work by returning the control lever to the neutral position, even if there occurs a failure, mixing noise or any other trouble in the control equipment, signal lines and so on which are located between the electric lever device and the controller.
More specifically, when the control lever of the electric lever device is operated a predetermined amount from the neutral position in the above drive control system, an operation signal depending on such an input amount of the control lever is output from the potentiometer and applied to the controller. The calculating means in the controller calculates a control signal corresponding to the operation signal, and a drive signal corresponding to the control signal is output from the output means in the controller, followed by being applied to the electro-hydraulic converter for conversion into a hydraulic signal. In this case, since the solenoid switching valve does not cut off the hydraulic pilot pressure, the hydraulic pilot pressure from the hydraulic source is supplied through the electro-hydraulic converter to one of the hydraulic pilot operated sections of the directional control valve on both of its sides, whereupon the directional control valve is shifted. Correspondingly, a hydraulic fluid delivered from a hydraulic pump is supplied to the hydraulic actuator so that the hydraulic actuator is driven to operate an associated working member, e.g., one of a boom, an arm, a bucket, an upper swing, and a lower travel device.
On the other hand, when the control lever of the electric lever device is in the neutral position, this condition is detected by the neutral position detecting device and the corresponding detection signal is input to the solenoid switching valve which then cuts off the hydraulic pilot pressure between the hydraulic source and the electro-hydraulic converter. Accordingly, even if an error signal is produced in the controller, etc. due to a failure or due to mixing noise, causing the electro-hydraulic converter to malfunction, the directional control valve is supplied with no hydraulic pilot pressure and is held in its neutral position. Therefore, the hydraulic actuator is kept stopped. It is thus possible to prevent movement of the hydraulic actuator not intended by an operator and to ensure safety.